Support **DictConfig hparam serialization (#2519)

* change to OmegaConf API

Co-authored-by: Omry Yadan <omry@fb.com>

* Swapped Container for OmegaConf sentinel; Limited ds copying

* Add Namespace check.

* Container removed. Pass local tests.

Co-authored-by: Omry Yadan <omry@fb.com>

pytorch_lightning/core/saving.py

@@ -16,11 +16,9 @@ from pytorch_lightning.utilities.cloud_io import gfile, cloud_open
 PRIMITIVE_TYPES = (bool, int, float, str)
 ALLOWED_CONFIG_TYPES = (AttributeDict, MutableMapping, Namespace)
 try:
-    from omegaconf import Container
+    from omegaconf import OmegaConf
 except ImportError:
-    OMEGACONF_AVAILABLE = False
-else:
-    OMEGACONF_AVAILABLE = True
+    OmegaConf = None
 
 # the older shall be on the top
 CHECKPOINT_PAST_HPARAMS_KEYS = (
@@ -330,23 +328,27 @@ def save_hparams_to_yaml(config_yaml, hparams: Union[dict, Namespace]) -> None:
     if not gfile.isdir(os.path.dirname(config_yaml)):
         raise RuntimeError(f"Missing folder: {os.path.dirname(config_yaml)}.")
 
-    if OMEGACONF_AVAILABLE and isinstance(hparams, Container):
-        from omegaconf import OmegaConf
-
-        OmegaConf.save(hparams, config_yaml, resolve=True)
-        return
-
-    # saving the standard way
+    # convert Namespace or AD to dict
     if isinstance(hparams, Namespace):
         hparams = vars(hparams)
     elif isinstance(hparams, AttributeDict):
         hparams = dict(hparams)
+
+    # saving with OmegaConf objects
+    if OmegaConf is not None:
+        if OmegaConf.is_config(hparams):
+            OmegaConf.save(hparams, config_yaml, resolve=True)
+            return
+        for v in hparams.values():
+            if OmegaConf.is_config(v):
+                OmegaConf.save(OmegaConf.create(hparams), config_yaml, resolve=True)
+                return
+
+    # saving the standard way
     assert isinstance(hparams, dict)
-
-    with cloud_open(config_yaml, "w", newline="") as fp:
+    with open(config_yaml, 'w', newline='') as fp:
         yaml.dump(hparams, fp)
 
-
 def convert(val: str) -> Union[int, float, bool, str]:
     try:
         return ast.literal_eval(val)